Reference Fields and Probes
Goals
The Reference Fields and Probes Project develops methods and techniques for establishing continuous- wave electromagnetic (EM) reference fields and transfer probes for frequencies to 100 gigahertz. It maintains the capability to provide antenna, probe, and field measurements with international comparability and traceability to NIST in support of U.S. industry. Although most present applications utilize spectra in the 1 megahertz to 10 gigahertz range, systems such as automotive collision avoidance radars that operate up to nearly 100 gigahertz are being developed.
Customer Needs
Based on the principles of “one product, one technically valid international standard, one conformity assessment” (1998 MSL Strategic Plan), industry requires EM field measurement capabilities and transfer probes that are traceable to NIST in order to meet multinational compliance requirements and reduce barriers to worldwide acceptance of U.S. products. We address these needs with the following:
Reference Fields — Well defined EM reference fields are necessary for the calibration of antennas and probes. They are also needed for research and development to increase measurement accuracy and spectral range as will be necessary to support the future needs of U.S. industry and private test laboratories.
Field Probes — Accurate field probes are needed by government and industry to define EM field levels. U.S. defense and homeland security agencies rely heavily on EM systems for sensors and strategic communication. New probes need to be developed for the ever-expanding range of EM environments.
Probe Calibrations — Field probe calibrations are costly. Techniques to reduce calibration costs are needed, especially for applications that require multiple probes and frequent recalibration.
Technical Strategy
We maintain an integrated effort both to generate standard reference fields and to develop the probes required for their accurate measurement. The two efforts complement each other and allow cross checking in order to reduce the uncertainties inherent in each effort as well as to transfer calibration capabilities to other test laboratories and facilities. As instrumentation and electronics achieve higher clock rates, measurements are needed at higher frequencies. We are working both to extend current techniques and facilities to higher frequencies and to develop new test methods to increase accuracy and reduce measurement costs. In this context, we plan to develop improved methods for measuring radio frequency (RF) emissions above 1 gigahertz.
Open area test site (OATS) facilities are accepted as standard sites for electromagnetic compatibility (EMC) emissions measurements. We are working on improvements of antenna characterization through tighter standards documentation, updated technology and enhanced methodology for EMC antenna measurements. We work closely with the American National Standards Institute (ANSI) and the Society of Automotive Engineers (SAE) to further improve their methods for EMC antenna measurements.
Fully anechoic chamber (FAC) facilities are accepted as standard sites for free-space measurements. Time-domain techniques are being studied as a way to measure the characteristics of these rooms and to improve the results obtained within. This type of chamber is also being evaluated for EMC product testing up to 40 gigahertz.
Closed test systems such as transverse electromagnetic (TEM) cells have been widely adopted for testing small antennas, sensors, and probes, but are normally limited by geometrical constraints to frequencies below 1 gigahertz. We are currently constructing a new closed-cell system that utilizes a co-conical geometry that can be used to test such devices up to 45 gigahertz. The test volume of this system is large enough to calibrate several probes at once. EM modeling and analysis using numerical techniques such as finite-difference time-domain are used to predict system performance for multiple probes.
 Co-conical field generation system being assembled at
|
Measurements performed by use of different equipment and facilities such as OATS, TEM cells, FAC and semi-anechoic facilities often yield different results. We will focus on systematically investigating methods to reduce these variations and improve agreement within the U.S. industrial community. 27 We will provide technical information and guidance to standards organizations to help correlate measurements between various EMC test facilities. We will also cooperate with the national test laboratories of our international trading partners to perform round-robin testing and comparison of various standard antennas and probes. This assures international agreement in their performance and reduces the uncertainties in the areas of metrology that affect international trade.
Accomplishments
- Probe Calibrations — Calibrations were performed on probes/antennas for several companies and/or government agencies covering the frequency range of 10 kilohertz to 45.5 gigahertz by use of TEM cell and anechoic chamber test facilities. Field levels varied from 1 to 200 volts per meter.
- Co-Conical Field Generation System (CFGS) — A 10 megahertz to 40 gigahertz RF probe test facility for the U.S. Air Force is nearing completion. The final machining of the test cell was completed in July 2006 and delivered to NIST for testing and full system integration. The CFGS is made of two major subsystems. The first is a harmonically pure RF generating system capable of producing 25 watts of RF power out of a single RF connector from 10 megahertz to 40 gigahertz. This subsystem involved the design of new RF components that are now standard product offerings from two companies. The power delivery subsystem feeds the power into the second subsystem: a broadband TEM transmission system and termination that can generate high-intensity fields and calibrate probes faster, and with comparable uncertainties, than conventional anechoic chambers.
- Electric and Magnetic Field Probe — A loop antenna with integrated photonics and controls to simultaneously measure electric and magnetic fields at levels up to 1 kilovolt per meter has been developed.